A platinum-rhodium catalyst is utilized in the Andrussow process, for the production of hydrocyanic acid or hydrogen cyanide (HCN). In the Andrussow process, methane, ammonia, and oxygen are reacted by passing that gas mixture through a platinum alloy gauze pad. The reaction takes place at about 1000.degree. C., and the heat liberated enables it to be self-sustaining. HCN formation proceeds according to one or other of the following reactions: EQU CH.sub.4 +NH.sub.3 +3/2O.sub.2 .fwdarw.HCN+3H.sub.2 O EQU NH.sub.3 +CH.sub.4 .fwdarw.HCN+3H.sub.2
The mixed gases are preheated to about 500.degree. C., and the methane:ammonia mixture ratio is in the range 0.8 to 1.2:1.
Sufficient air is added to provide 25 to 40 percent of the theoretical requirement to oxidize the methane and ammonia. Reactors are similar to ammonia oxidation converters, although usually smaller in diameter. Since the gauze operates at about 1000.degree. C., its radiant heat causes the converter walls to be hotter than in ammonia oxidation, and some ammonia is lost from the reaction by predecomposition. Some carbon monoxide and nitrogen are also formed by undesirable side reactions.
In hydrocyanic acid production, the catalyst pack consists of flat woven gauze. The conventional flat woven gauze is typically made with 80 mesh per inch and 0.003" wire. UK patent GB 2062486 B discloses the use of a system where the diameter of the wire is reduced from the front to the back of the flat woven gauze pack resulting in maximum conversion efficiency with minimum precious metal content and therefore also minimum metal loss from the catalyst. Other recent disclosures include U.S. Pat. No. 4,863,893 which claims improved catalyst "light-off" by the use of a high surface area per unit area of catalyst by the deposition of fine platinum particles onto the surface of the flat woven gauze. U.S. Pat. No. 4,863,893 is a variation of the technology of U.S. Pat. No. 3,470,019 which had a different method of deposition. Patent application EP O 364 153 Al claims the use of a flat knitted fabric of precious metal for the oxidation of ammonia to nitric oxide. Recently, a flat woven gauze of 70 mesh, 0.003" wire diameter was introduced to the market place.
The catalyst pack of elements or sheets of gauze of precious metals and its use to catalyze ammonia is disclosed in U.S. Pat. No. 4,412,859, U.S. Pat. No. 4,497,657 and U.S. Pat. No. 4,526,614, all of which are hereby incorporated by reference, in toto,
By curve to flat ratio is meant the ratio of that portion of an element of a catalyst (a sheet or gauze etc.) that is not in the base plane of the element to the portion that is in that plane. When curved, it is the ratio of curved section to the flat section.